The invention is based on a three-stage electromagnet of a 3/3-way magnetic valve for fuel injection systems. An electromagnet of this type actuates the directional control valve slide counter to the force of the restoring spring; in its non-magnetized position, it connects the working cylinder, which for instance actuates the injection quantity control member, with a return line, while in a middle position of the armature or slide it blocks a connection with the work cylinder, and in the end position of the armature or slide, that is, when it is supplied with maximum current, it conconnects a hydraulic source with the work cylinder, which then acts in the direction of increasing injection quantity. The problem here is the triggering of the middle position, because this position is not determined by any stop for the armature or slide but instead is a function of a mean value of the electric current supplied to the magnet. To this end, the magnetic forces corresponding to the mean value range of the electric current must be unequivocally distinguished from the limiting forces, that is, zero force and maximum force, corresponding to the limit currents of zero and maximum current. In the ideal case, the hydraulic flow should correspond to the electric current but with the middle position as zero; that is, a maximum electric current would mean maximum hydraulic current in the direction of increasing injection quantity, and minimum electric current (zero current) should correspond to maximum hydraulic flow in the direction of decreasing injection quantity.
In controlling slide valves it is typical to use magnets in which the core housing has a central core about which the winding is disposed coaxially with the direction of movement of the slide, so that the magnetic flux flows via this central core and the outer parts of the housing as well as the armature. Two principles of application are known for the manner in which the force flows. By one of these principles, the armature is provided with a central bore and is disposed so that it slides on the core, and when magnetically excited it plunges into an opening provided therefor between the outer yoke parts. The armature has an internal cone on the side facing the yoke, so that if the restoring spring has a linear characteristic, an adjustment of the armature or variation of the magnetic force that is proportional to the current intensity is attainable. Such proportional magnets, which can be adjusted as a function of a characteristic curve, have the disadvantage that of the two air gaps (between the core and armature and between the core and housing), only one--for instance, between the core and the housing--contributes to a generating force; the other air gap requires nonuseful magnetic energy. Since the magnetic pole having a first polarity (for instance positive) is supported between the poles of a second polarity, the stray flux at a given outer diameter is relatively high. Because of asymmetry, friction losses also occur between the armature and the core, besides the fact that such a magnet is relatively vulnerable to dirt. A further disadvantage is that one or two ring seals for sealing the magnet from the outside must be provided, namely between the valve region and the winding, to prevent oil from getting into the cup of the magnet, which receives the winding and has openings on its end face remote from the valve through which the connection cable passes. Usually, another ring seal must also be provided between this magnet cup and the housing receiving the magnet.
The other application principle of magnets of this kind actuating control slides relates to flat pole magnets, in which the armature does not plunge radially inward as in the case of the proportional magnets. As a result, however, characteristic curve influence cannot be exerted and the characteristic curve of the stroke has a quadratic course instead; that is, the magnetic force decreases by a power of two with increasing distance. This disadvantage can be compensated for only by a complex and expensive nonlinear characteristic electrical curve on the valve side for controlling flow; however, the system remains asymmetrical and thus accuracy of control is much more difficult to achieve. Although the vulnerability to dirt is much less in comparison with the first application principle above, and the additional friction forces are sharply decreased as well, namely by eliminating the need to guide the armature centrally on the core, the disadvantage of having to provide twice as many seals remains.